1. Field of the Invention
The present invention relates to a superconductor for magnetic field shielding which shields magnetic fields using superconductors.
2. Prior Art
As a magnetic field shielding using superconductivity the first class superconductor as type I or the second class superconductor as type II has been used depending on the intensity of a magnetic field. The superconductor for magnetic field shielding comprising the first class superconductor uses perfect diamagnetism (Meissner effect), a property of superconductivity. The superconductor cannot shield intense magnetic fields since its critical magnetic flux density is low. The superconductor for magnetic field shielding comprising the second class superconductor utilizes a mixture of the superconductor state and the normal conduction state, and its critical magnetic field is separated into the upper and lower critical magnetic flux densities. Since the upper critical magnetic flux density is extremely high, the superconductor comprising the second class superconductor can be used to shield intense magnetic fields. Since such a superconductor is thicker, its shielding effect increases. Therefore, a thick superconductor is used to shield intense magnetic fields. However, due to local flux flow such a thick superconductor may generate heat, resulting in reduced shielding effect and secondary harmful effects. Accordingly, when the above-mentioned second class superconductor is used to shield intense magnetic fields, relatively thick sheets or tapes of the superconductor are laminated in many layers or alternately laminated with aluminum or copper sheets. Since these two laminated types are thick and weighty when they are made to shield intense magnetic fields, they are not satisfactory in practical use.
Furthermore, the former laminated type is low in stability since it easily causes flux jumping (magnetic fluxes enter and move into the central section of the superconductor increasing the temperature of the superconductor and causing a catastrophic phenomenon of the entry of more flux) due to quick excitation even in weak magnetic fields. The latter laminated type has a better construction than the former one since aluminum or copper sheets are effective in cooling. Nevertheless, the characteristics of the superconductor are not fully realized even when the construction of the latter type is used. Moreover, since it was understood that a higher shielding effect was obtained from a thicker superconductor (the thickness of the superconductor was thought to be proportional to the magnetic field shielding effect), a thick superconductor was thought to be more advantageous than laminated thin superconductor layers to reduce production cost and production processes provided that a means for preventing the above-mentioned heat generation was taken.